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  1. #1
    Active Member Two Rings Dr Awesome's Avatar
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    Aug 21 2017
    AZ Member #
    405435
    Location
    Minneapolis

    N75 17956 code troubleshooting

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    Hello,
    I am trying to track down an underboost condition in my stage 3 s4. It has only been boosting (slowly) to 15 psi when it should spike to 24 and taper to 22.

    To make a long story short, I found and fixed two boost leaks, replaced 710p diverter valves with 034 diverter valves, and bought a new n75 valve.

    After all of this I still had no luck, the car is still struggling to make 15 psi. While fixing the boost leaks, scanning with vcds did not show any thing. After replacing the n75 I get code 17956 every time I try to go into boost. Clearing the code and rescanning it while idling it does not come on, but it returns after trying to go to boost.

    This code would indicate a faulty n75 or a break in the wiring, but I have checked the wires, they both have continuity. The resistance on the new n75 is 32.5 ohm (within spec) and I ran the n75 output test with vag com and could hear the n75 clicking.

    Any ideas on next steps? When I can get around to it I'm going to smoke test this thing on a lift to find any other leaks, I can't hear or find any with soapy water from the top.

    Sent from my SM-G950U using Audizine mobile app

  2. #2
    Veteran Member Four Rings vavJETTAw36's Avatar
    Join Date
    Jul 04 2012
    AZ Member #
    96276
    Location
    Woodbridge, VA

    Check your wiring.

    The n75 is activated by the ecu throwing a ground signal, I believe that is pin 2 on the n75 harness (green yellow wire) and pin 104 on the ecu. The other pin 1 is a common 12v for many components.

    I suggest checking the pin 1 for 12v while shaking the wiring. If that is good, check for continuity between pin 2 and ecu pin 104.

    If that checks out too you could run your own ground for the sake of testing and or 12v.

    When the ecu wants to control boost it “breaks” ground.


    Sent from my iPhone using Tapatalk

  3. #3
    Veteran Member Four Rings CELison's Avatar
    Join Date
    Feb 23 2010
    AZ Member #
    55053
    Location
    Lehigh Valley

    Wastegate nuts could have backed off some and lost preload on your gates. I’ve had that happen on every aftermarket turbos I’ve ran.

  4. #4
    Active Member Two Rings Dr Awesome's Avatar
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    Aug 21 2017
    AZ Member #
    405435
    Location
    Minneapolis

    Thank you both for the quick replys! Both wastegates crack at the same time ~6 or 7 psi. I will check pin 1 for 12 v while shaking the wiring, both wires have continuity from the ECU to the plug connection. Thanks again for the quick responses!

    Sent from my SM-G950U using Audizine mobile app

  5. #5
    Veteran Member Four Rings vavJETTAw36's Avatar
    Join Date
    Jul 04 2012
    AZ Member #
    96276
    Location
    Woodbridge, VA

    N75 17956 code troubleshooting

    Quote Originally Posted by Dr Awesome View Post
    Thank you both for the quick replys! Both wastegates crack at the same time ~6 or 7 psi. I will check pin 1 for 12 v while shaking the wiring, both wires have continuity from the ECU to the plug connection. Thanks again for the quick responses!

    Sent from my SM-G950U using Audizine mobile app
    If that checks out, go ahead and use alligator clips or back probe to hook up a 12v and ground source. Keep in mind if this works you’ll be overboosting since the car will no longer have control of the n75.

    If you continue to get the code you might have something wrong with your new n75 or ecu.


    Sent from my iPhone using Tapatalk

  6. #6
    Active Member Two Rings Dr Awesome's Avatar
    Join Date
    Aug 21 2017
    AZ Member #
    405435
    Location
    Minneapolis

    Quote Originally Posted by vavJETTAw36 View Post
    If that checks out, go ahead and use alligator clips or back probe to hook up a 12v and ground source. Keep in mind if this works you’ll be overboosting since the car will no longer have control of the n75.

    If you continue to get the code you might have something wrong with your new n75 or ecu.


    Sent from my iPhone using Tapatalk
    I tested pin 1 (blue with green stripe) for 12 volts with the key turned to the second position(radio on car not running) and was unable to get any voltage reading. I do get continuity, however. Does this mean I need to find and fix a break in my blue green wire?

    Sent from my SM-G950U using Audizine mobile app

  7. #7
    Senior Member Three Rings insomniacxp1's Avatar
    Join Date
    May 10 2010
    AZ Member #
    58820
    My Garage
    01.5 B5 S4
    Location
    Rochester, NY

    You disconnected ecu connector and tested pin on ecu to pin on n75, both harness side?
    01.5 Imola S4. Stage 3 F21's/To much to list

  8. #8
    Active Member Two Rings Dr Awesome's Avatar
    Join Date
    Aug 21 2017
    AZ Member #
    405435
    Location
    Minneapolis

    Quote Originally Posted by insomniacxp1 View Post
    You disconnected ecu connector and tested pin on ecu to pin on n75, both harness side?
    Not quite, but your comment got me thinking that I was doing something wrong haha. I definitely did not check for 12v correctly the first time around...

    I tried again using what I believe is the correct method. Full disclosure I am learning some things as I go with this car... Pictures are worth a thousand words, can somebody please let me know if I'm testing for 12v correctly? I was able to read 3.5 with the key in the second position, shaking wiring harness did not change the voltage:
    https://imgur.com/edit?deletehash=X8...bum_id=mCurYso



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